In a conventional power circuit for a battery, in order to reduce a capacity of a capacitor while ensuring an output electric power in start-up and power-running of an electric motor, a capacitor (or a high output battery having a small capacity) is connected in series with a battery of 12 V, and a DC/DC converter having a small capacity is added for the purpose of shifting an electric energy between the capacitor and the battery. In this power circuit for a battery, the DC/DC converter is operated for boosting in power running of the electric motor, whereby a desired output can be obtained with the capacitor having a relatively small capacity (or a battery having a relatively small capacity) (refer to JP 2002-218667 A (FIG. 3) for example).
In the conventional power circuit for a battery, there is a problem in that if an idle-stop operation (stop/start-up operation) is continuously carried out, then the recharging for a capacitor group connected in series with a battery group becomes insufficient, so that it becomes impossible to supply a sufficient electric power to a motor through an inverter, and hence a predetermined start-up operation by the motor of a vehicle cannot be carried out. A predetermined start-up operation means an operation for increasing the rpm of an engine from a stop state to an idle running state (engine rpm of about 800) with the motor.
In addition, there arises a problem in that a sufficient motor output can not be obtained due to insufficiency in voltage of the capacitor, and hence the start-up by the motor can not be carried out.
In addition, if the idle-stop operation is continuously carried out, then a sufficient period of time cannot be secured to recharge the capacitor with electricity. Thus, the capacitor voltage at the start-up operation takes various values, and the start-up operation is carried out in this state. Then, there arises a problem in that if the DC/DC converter is operated with a fixed output independently of the capacitor voltage, then the efficiency of the overall battery drive circuit system including a battery, a capacitor, and a DC/DC converter deteriorates depending on the capacitor voltage values. The deterioration of the efficiency causes an increase in a quantity of heat generation by the overall system. In particular, there is a concern about reduced life of the capacitor due to rise in temperature caused by the heat generation, and the heating of other apparatuses.
In addition, due to internal resistances that exist in the capacitor and the battery, there is a possibility that in outputting a large electric power at the engine restart-up or the like, a voltage drop due to the internal resistance exerts adverse influences on other on-vehicle apparatuses. In particular, when a charge voltage of the capacitor is low or a state of charge (SOC) of the battery is low, there is a problem in that a battery current required for restart-up of an engine is increased to decrease a battery output voltage.
In regeneration of energy, a series-connected body of the battery and the capacitor is charged with the energy generated by the electric motor. The permissible output electric power density of a lead acid battery is low, about 100 to 200 W/kg, and allowable input electric power density is even lower. For this reason, charge current during regeneration is determined on the basis of the allowable input current of the battery. Note that the allowable input electric power density of the battery is proportional to the allowable input current since the voltage of the battery is nearly fixed. Thus, the high speed charging characteristics of the capacitor can not be utilized, and hence it is necessary to limit the regeneration output of the electric motor.